Role of Strain for Cover Geometry in Valley and Ridge Province of West Virginia
Dan A. Billman, Matthew A. Johnston, William M. Dunne
Silurian to Pennsylvanian cover rocks were deformed during the formation of blind Alleghenian duplexes in the underlying Cambrian-Ordovician carbonates in the Valley and Ridge province of the central Appalachians. Possible geometric responses by the cover to duplex formation include (1) macroscale folding and faulting, (2) mesoscale folding, faulting, and cleavage formation, and (3) microscale penetrative deformation. The present study examines the contribution to cover geometry of microscale deformation in terms of strain magnitude and timing.
Within the study area, samples for strain determination were collected along six section lines, which extended from the Little North Mountain thrust in the east to the Alleghenian structural front in the west. Along each section line, 20-25 oriented samples of well-sorted, quartz-rich, framework-supported sandstones were collected from seven levels between the basal Silurian and basal Mississippian. The use of seven levels enabled sampling in both cover anticlines and synclines and allowed assessment of strain variation with stratigraphic level. Strain determination was performed on oriented sample thin sections using the normalized Fry and Rf/phi methods. These strain data are presented on six new cross sections that were constructed from new field data and existing geologic maps.
These strain data were used to determine whether microscale deformation (1) made little contribution to cover deformation, (2) occurred as a layer-parallel shortening preceding the emplacement of underlying horses, (3) occurred as only local strains in response to flexural flow during folding, or (4) occurred as flattening during emplacement of underlying horses.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.